1. Field of the Invention
The invention relates to a method of retrieving data requested by a host from a first memory divided into allocation units.
The invention further relates to an apparatus for retrieving data requested by a host from a first memory, comprising means for receiving data from the first memory, the first memory being divided into allocation units; a second memory; and a central processing unit.
The invention also relates to a system for reproducing audio-visual information.
2. Description of the Related Art
Such a method, apparatus and system are known from U.S. Patent Application Publication NO. 2001/0021983, corresponding to U.S. Pat. No. 6,725,395. This patent proposes a method of relocating defective sectors on a harddisk platter to sectors in a spare area on the platter, and an apparatus for performing this method. According to the method presented in this patent, the spare areas are located outside the data area, either on the outside or inside of the disk platter.
When retrieving data from a group of sectors on the harddisk platter, in which group certain sectors are defective and therefore relocated to sectors in a spare area, the reading head has to switch from an area on the disk platter where the group of sectors is located to either one of the spare areas. Although the patent proposes embodiments for reducing the performance penalty incurred by switching of the head, there will always be a performance penalty when retrieving data from said group of sectors. This is because the reading head will always have to make a switch (or “sweep”) from the normal data area to the sectors in the spare area.
This is a disadvantage of the prior art. Harddisk drive systems are more and more used for audio-visual applications, which require optimal real-time performance and bandwidth of such systems. Especially when multiple streams of audio-visual information are streamed from the harddisk drive system—or any other kind of memory device—performance penalties that occur during streaming may cause buffer underflow. When retrieving a stream of audio-visual information for playback, this may result in hiccups in the presentation of the stream on screen. This not only goes for performance penalties caused by out-of-bound relocation of defective sectors, but also for so-called “soft defects”, sectors of which data has to be read multiple times before obtaining the correct data.
Furthermore, it is known from practice that files, either data files or streams of audio-visual information, may be stored in fragments on the harddisk platter. Switching from one fragment to another may incur a performance penalty as well, the reading head has to switch from one area on the harddisk platter to another. During this sweep, no data is read.